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Abstract— Achieving both the precise pinching and compliant
grasping capabilities is a challenging goal for most robotic hands
and grippers. Moreover, an active transition between the
pinching pose and the grasping pose is difficult for limited
degrees-of-freedom (DOF) hands or grippers. Even when using
high DOF robotic hands, it requires a substantial amount of
control effort and information from tactile sensors. In this paper,
a 3-finger, 5-DOF adaptive gripper with active transition
capability is presented. Each finger is composed of a minimum
number of components using one rigid link, one belt, one
fingertip frame and one motor for flexion motion. This simple
and unique finger structure enables precise parallel pinching and
highly compliant stable grasping with evenly distributed
pressure. The other two motors are used for fingertip angle
adjustment and change of the finger orientation respectively.
Thorough kinematic and force analysis with detailed
descriptions of the mechanical design clearly shows controllable
transition property and stable grasping performance. The
experiments including the grasping force and pressure
measurement verify the performance of the proposed gripper
and prove the practical usefulness for real-world applications.
I. INTRODUCTION
In real-world situations, utilizing tools to accomplish a task
often requires a large variety of grasp types. For instance, in
order to use a small screwdriver, the user pinches the handle
using 2 or 3 fingertips and changes to a compliant grasping
pose to apply high torque. After using it, the user returns to the
pinch pose to lay it down gently on the table. This simple
example not only requires the extreme types of grasping -
precise pinching and compliant grasping - but also the smooth
and stable transition between the grasping modes.
To categorize and utilize the functions of the human hand,
many studies on grasping taxonomies have been investigated
[1][2]. These studies identify greater than 20 or 30 distinctive
grasp types, which implies the versatility of the human hand.
To realize similar capability, various types of high
degrees-of-freedom (DOF) human-like robotic hands have
been developed [3][4][5] aiming at the performance of a
human hand having 15 joints and more than 20 DOFs. They
contain complex and expensive mechanisms and sensors and
require complicated motion planning and control, all which
hinders their widespread use in practical applications and
*Resrach supported by NAVER Labs. Co. 1.Yong-Jae Kim, is with Korea University of Technology and Education
(Koreatech), Cheonan-City, Rep. of Korea (corresponding author to provide
phone: +82-41-560-1424; fax: +82-41-564-3261; e-mail: yongjae@
koreatech.ac.kr). 2Hansol Song and 3Chan-Young Maeng are with Korea University of Technology
and Education (Koreatech), Cheonan-City, Rep. of Korea (e-mail:
[email protected], [email protected]).
further commercialization.
Based on the grasp taxonomy in [1], grasping can be
categorized into precision grasps and power grasps. One of the
important grasp modes in the precision grasp is pinching
objects with parallel fingertip surfaces, which will be referred
to as parallel pinching in this paper. One of the important
aspects of the power grasp is adaptability to irregularly shaped
objects. Therefore, the power grasp focusing on compliance to
the arbitrary shapes will be referred to as compliant grasping in
this paper. Focusing on compliant grasping capability with
reduced mechanical complexity and a fewer number of
actuators, various approaches including under-actuated
mechanisms [6], synergy based mechanisms [7], and soft and
flexible mechanisms [8] are being continuously investigated.
Their inherent adaptability to arbitrarily shaped objects
enables simplified control strategy and safe interaction with
objects and environment. The iHY Hand [9] and Yale
OpenHand [10], which utilize under-actuated monolithic
finger structures, showed remarkable robustness and
adaptability with relatively simple mechanisms. Nonetheless,
this inherent compliance may reduce the abilities of precise
manipulation and the ability to change the grasping mode. To
overcome these, many control schemes such as the extrinsic
dexterity which actively utilizes the environment and gravity
[11] and adoption of human finger motions [12] are being
studied. Various novel mechanisms have been introduced to
increase the performance of under-actuated hands. For
instance, the Velvet Fingers utilizes conveyor systems as
active contact surfaces to achieve in-hand object rotation and
translation capability [13]. Another example is the use of
deformable rubber bags filled with fluid to enhance grasping
capabilities with various grasping modes [14].
Yong-Jae Kim,1 Member, IEEE, Hansol Song2, and Chan-Young Maeng3
BLT Gripper: An Adaptive Gripper with Active Transition
Capability between Precise Pinch and Compliant Grasp*
Fig. 1. Proposed BLT Gripper (Belt and Link actuated Transformable
adaptive Gripper with active transition capability).
IEEE Robotics and Automation Letters (RAL) paper presented at the2020 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS)October 25-29, 2020, Las Vegas, NV, USA (Virtual)
Copyright ©2020 IEEE
To achieve both precise pinching and compliant grasping
capability through mechanical design, several universal
grippers have been proposed. The Sarah hand [6] and its
commercial product Robotiq 3-Finger Gripper [15] have the
combined structure of under-actuated mechanism for the
compliant grasping and the parallelogram linkage for the
precise parallel pinch. The Robotiq Adaptive Gripper [16],
widely used in research field recently, has an elegant
mechanism to realize the two grasping modes with a
significantly simpler mechanism. The detailed explanation is
provided in Section II. These two grasping modes also can be
accomplished by using coupling tendons and actuation
tendons as implemented in the Velo gripper [17]. The
ParaGripper composed of multiple parallel 4-bar links and
parallel fingertips also achieved both the parallel pinching and
power grasp as well as in-hand manipulation capability [18].
Although the active transition between the different grasping
modes is also a critical issue as aforementioned, these grippers
do not provide active transition ability. Thus, finding the
proper gripper mechanisms which provide various grasping
modes enabling stable and active transition between the modes
is still to be further explored.
As an effort of this, this paper proposes a 3-finger 5-DOF
robotic gripper with an active transition capability as shown in
Fig.1. Each finger is composed of a minimum number of
components; one rigid actuation link, one belt, one fingertip
frame, and one motor. The simple and unique finger structure
enables precise parallel pinching and highly complaint stable
grasping with evenly distributed pressure. The remainder of
this paper is organized as follows. Section II introduces the
concept of adaptive grippers and the idea of the proposed
gripper. In Section III, kinematic analysis and the force
distribution analysis are described. In Section IV, the detail
design and implementation are presented. Section V presents
the experimental reports the evaluation results, and Section VI
provides conclusion and future work.
II. BASIC CONCEPT OF ADAPTIVE GRIPPERS
A. Concept of Adaptive Grasping Mechanisms
Fig.2 (a) and (b) illustrate the representative examples of
under-actuated finger mechanisms. The triangular linkage
structure in Fig.2 (a), a simplified illustration of the
under-actuated finger in [6], has 3 DOFs and 1 actuation.
When the actuation link at the bottom rotates, the structure
conforms to the object and produces distributed contact forces.
This linkage structure is suitable for high payload grasping,
but it may reduce the range of motion. Fig.2 (b) shows a
similar under-actuated mechanism but with a more human-like
mechanism using tendons. The tendons goes around the
free-rotating pulleys (blue circles) centered at the revolute
joints and the tension of the tendon produces a constant torque
distribution to all joint. This torque distribution to each joint is
clearly determined by the radius of the pulleys [19]. For
simplicity and more compliance, the revolute joint can be
replaced with a flexure and the tendon routing can be
simplified as shown in the right most figure of Fig. 2(b)
[9][10][12]. Another approach is to utilize the material
flexibility. Fig. 2(c) shows the well-known Fin-Gripper from
FESTO Co. which comprises 2 flexible frames and multiple
ribs [20]. It continuously wraps around the object and applies a
well-distributed pressure rather than point contacts, which
provides more stable and safe grasping. There are many
mechanisms with similar concepts [21][22]. For more
compliance and safely, soft materials and air pressure can be
used as shown in Fig. 2(d). The tradeoff for these advantages
can be a sacrifice in precision of the grasping position and a
lower payload. The ideal goal for compliant grasping is to
provide uniform force or pressure regardless of the object size
and shape.
B. Principles of the Transition between the Parallel Pinch and
Compliant Grasp
By combining the under-actuated structure of Fig. 2(a) and
parallel 4-bar linkages, both the complaint grasping and
parallel pinching can be achieved as shown in Fig. 3(a), where
the green colored links represent the parallel 4-bar linkages.
The behavior depends on the position of contact point in
relation to the 𝑝𝑐𝑟 which is the intersection of point of the two
extended lines of the vertical links of the triangular linkage
(black colored links). If the contact with the object occurs
above the intersection point 𝑝𝑐𝑟 , the fingertip maintains the
Fig. 2. Conceptual examples of adaptive grasping. (a) An under-actuated
finger composed of multiple trapezoidal 4-bars. (b) An under-actuated finger
using joints and tendons. (c) Complaint finger mechanisms using flexible
structures and flexures. (d) A soft finger actuated by air pressure.
(a)
(c) (d)
(b)
Actuation cable
Freely rotating pulleys
Soft joints
Flexible frames
Ribs
Actuation link
Actuation cable
TrigonalLinkage structure
Actuation link
Air pressure
Silicone chamber
Fig. 3. Combined structures having parallel pinch and compliant grasp.
(a) Sarah hand [6]. (b) Robotiq Adaptive Gripper [16].
(a) (b)
stopper
𝑝𝑐𝑟
𝑝𝑐𝑟Critical line
Pinching region
Parallel 4-bars
Actuation link
Grasping region
𝑝𝑐𝑟
Critical line
Pinching region
Grasping region
Actuation link
𝑝𝑐𝑟
vertical pose by the support of the parallel 4-bars. Otherwise, if
the contact occurs lower than 𝑝𝑐𝑟 , the triangular linkage starts
grasping the object. Therefore, the horizontal line containing
𝑝𝑐𝑟 , named the critical line, determines the pinching region
and grasping region. For further explanation, refer to [6].
The Robotiq Adaptive Gripper achieves similar capability
with a significantly simpler mechanism. Basically, it acts as a
parallel 4-bar mechanism as shown in the left figure of Fig.
3(b), but the right vertical link is composed of two links with a
stopper (green and blue links). Similarly, the intersection point
𝑝𝑐𝑟 of the extended lines of the black and green links
determines the critical line. When the contact occurs above the
line, the stopper remain compressed and the mechanism works
as a parallel 4-bar. Otherwise, the stopper detaches and the
finger will start to bend to grasp the object as illustrated in the
rightmost figure of Fig. 3(b).
These two mechanisms accomplish the parallel pinching
and compliant grasping without increasing the number of
actuators. However, the grasping type is determined only by
the contact region, which means that once the grasping type is
determined it cannot be actively changed. Moreover, there are
practical issues. The mass of the passively moving parts, i.e.
the fingertip, the black link, and the green link in Fig. 3(b) can
cause undesirable vibration when the gripper is moving while
mounted on a manipulator. Also, more distributed contact
forces or contact pressure are required for more stable
grasping.
C. Basic Concept of the Proposed BLT Gripper
The analysis of the Robotiq Adaptive Gripper yields useful
intuition of adaptive gripping mechanics. Fig. 4(a) shows the
parallel pinching pose, where 𝑙𝑑, 𝑑𝑡, and 𝜓 denote the length
of the bottom link of the 4-bar, the distance between the
bottom line of the fingertip and the contact point, and the angle
between the rear link and the x-axis. When the actuation link is
fixed by the actuators, the fingertip receives 3 forces 𝑓𝑡, 𝑓𝑓,
and 𝑓𝑟, which indicate the contact force, the tensile force from
the front link, and the compressive force from the rear link,
respectively. 𝜓𝑡 indicates the direction of force 𝑓𝑟. From the
free-body diagram of the fingertip, the force and moment
equilibrium equations can be obtained as
Σ 𝐹x ∶ 𝑓𝑡 − 𝑓𝑟 cos𝜓𝑡 = 0,
Σ 𝐹y ∶ 𝑓𝑓 − 𝑓𝑟 sin𝜓𝑡 = 0, (1)
Σ 𝑀 ∶ 𝑓𝑓 𝑙𝑑 − 𝑓𝑡 𝑑𝑡 = 0.
From (1), 𝜓𝑡 direction of 𝑓𝑟 has following relationship as
tan𝜓𝑡 = 𝑑𝑡 / 𝑙𝑑 (2)
which means that force vector of 𝑓𝑟 is on the line between the
contact point and the distal joint of the rear link. In the case of
𝜓𝑡 > 𝜓, where the contact point is above the critical line as
shown in Fig. 4(a), the rear link is in contact with the actuation
link via the stopper and the finger maintains a parallel pose.
On the other hand, if 𝜓𝑡 < 𝜓, where the contact point is under
the critical line as shown in Fig. 4(b), the distal joint of the rear
link moves along an arc-shaped path and the finger starts to
grasp the object compliantly. From the behavior of the Robotiq
gripper, we can find that the role of the rear link is guiding the
fingertip along an arc-shaped curve, and therefore the rear link
can be replaced with a slot. Another notable fact is that the
front link always receives tensile force. A flexible belt can
substitute for the front link and may increase the even
distribution of contact forces.
Considering the above analysis, a new finger mechanism is
proposed as shown in Fig. 4(c). The front link and the rear link
were replaced by a belt and a slot respectively. In order to
secure the large parallel pinching region when pinching
objects and to make the mechanism simple, the slot was
designed to have a straight shape along the actuation link.
Instead, to actively transit to the compliant grasping mode, a
slider adjusting the belt length was added. As shown in Fig.
4(d), by reducing the belt length, the fingertip closes around
the object and the contact point moves to the compliant
grasping region (blue area). Then, the finger compliantly
grasps the object. Fig. 4(e) shows its mechanical design. The
actual actuation link was designed as a curved shape to avoid
contact with the belt when grasping small objects. To keep the
fingertip straight when not in contact, a spring is placed
between the fingertip and the actuation link. As the passively
moving parts are only the lightweight belt and the fingertip, it
does not cause unwanted vibration when moving on a
manipulator.
III. KINEMATICS AND FORCE DISTRIBUTION
A. Kinematics at the Compliant Grasping Pose
Let us assume that the finger is grasping a circular object as
shown in Fig. 5(a). In most cases, contact occurs in three areas:
the fingertip, the belt and the palm. As indicated in Fig. 4(c)
(also in Fig. 5(a)), the points 𝑝𝑜 , 𝑝𝑏1 , 𝑝𝑏2 , 𝑝𝑎1 , 𝑝𝑎2 , and 𝑝𝑡
(a) (b)
(c) (d) (e)
Fig. 4. (a) and (b) Free body diagrams of the fingertip of the Robotiq
Adaptive Gripper. (c) and (d) Proposed finger mechanism. (e) Mechanical
design of the proposed finger.
𝑓𝑡
𝑓𝑟𝑓𝑓
𝑓𝑡2
Actuation link
𝑓𝑡
𝑙𝑑
𝑑𝑡
𝑓𝑓
𝑓𝑟
Front link
Rear link
𝜓𝜓𝑡
Critical line
Arc shapedslot
𝑑𝑏
𝑝
𝑝𝑎2
Complaint grasping region
𝑝𝑏1𝑝𝑜 𝑝𝑎1
𝑙𝑎
𝑙𝑑
𝑙𝑡
𝑑𝑏
𝑝𝑎2𝑝𝑏2
Belt
Actuation link with
a straight slot
Critical line
slider
Belt
Straight slot
Actuation link
Gear train
Extension spring
Slider
𝑓𝑡
𝑑𝑡
denote, the center point of the palm, the proximal and distal
points of the belt, the proximal and distal points of the
actuation link, and the contact point with an object at the
fingertip, respectively. , 𝑑𝑏 , 𝑙𝑎 , and 𝑙𝑡 mean the distance
between 𝑝𝑜 and 𝑝𝑏1 , the length of the belt, the length the
actuation link, and length of the contact surface of the fingertip,
respectively. The points 𝑝𝑐1, 𝑝𝑐2, and 𝑝𝑐𝑥 in Fig. 5(a) indicate
the lower and upper border points of the contact area of the
belt and the point at the horizontal line, respectively. The
angles determined by the center of the object and the points 𝑝𝑜,
𝑝𝑏1, 𝑝𝑐1, 𝑝𝑐𝑥 , 𝑝𝑐2, 𝑝𝑏2 and 𝑝𝑡 are named as 𝜙1, 𝛼, 𝛽, and 𝜙2,
respectively, and calculated as
𝜙1 = atan( /𝑟), 𝜙2 = atan(𝑑𝑡/𝑟),
𝛼 = 𝜋/2 − 2𝜙1, 𝛽 = (𝑑𝑏 − − 𝑟𝛼 − 𝑑𝑡)/𝑟.
For the convenience of calculation, let us define
𝛾 ≡ 𝛽 + 2𝜙2.
From these, the positions of 𝑝𝑡 , 𝑝𝑏2, and 𝑝 are calculated as
follows:
𝑝𝑡 = [ 𝑟 𝑐𝑜𝑠 𝛾 , 𝑟 + 𝑟 𝑠𝑖𝑛 𝛾 ]𝑇 (3)
𝑝𝑏2 = [𝑟 cos 𝛾 + 𝑑𝑡 sin 𝛾 , 𝑟 + 𝑟 sin 𝛾 − 𝑑𝑡 cos 𝛾]𝑇 (4)
𝑝 = [ (𝑟 + 𝑙𝑑) cos 𝛾 + 𝑑𝑡 sin 𝛾 , 𝑟 + (𝑟 + 𝑙𝑑) sin 𝛾 − 𝑑𝑡 cos 𝛾]𝑇(5)
From (5), the actuation angle shown in Fig. 5(a) can be
represented as
(𝑑𝑡) = atan (𝑝𝑠𝑦
𝑝𝑠𝑥−𝑤−𝑙𝑑 ) (6)
where 𝑝 𝑥 and 𝑝 𝑦 are x and y elements of the 𝑝 respectively.
Note that the positions (3), (4), and (5) and the angle (6) are
functions of 𝑑𝑡 . By solving the following optimization
problem, the equilibrium point of 𝑑𝑡 can be found.
max 𝑑𝑡∈[0,𝑙𝑡]
(𝑑𝑡). (7)
Fig. 5(b) illustrates all the possible fingertip poses satisfying
𝑑𝑡 ∈ [0, 𝑙𝑡] (dotted lines) and the equilibrium pose (solid lines).
The trajectory of 𝑝 according to 𝑑𝑡 represented by a dotted
black curve has a convex shape, which confirms that the
equilibrium pose can be fully determined by kinematic
information. In order to guarantee the grasping stability, two
facts should be satisfied. First, the contact should be in the
fingertip surface, i.e. 𝑑𝑡 ∈ [0, 𝑙𝑡]. Second, contact forces and
pressure should have positive values. The following
subsection explains these.
B. Force Distribution at the Compliant Grasping Pose
Unlike other compliant grippers, the force and pressure
distribution of the proposed gripper can be clearly formulated
as it has a simple structure using the belt for uniform pressure
distribution. Fig. 5(c) illustrates the free body diagram of the
fingertip, where 𝑓𝑎 and 𝑇𝑏 mean the force from the actuation
link and the belt tension respectively. The angles of the force
vectors of 𝑓𝑎 and 𝑇𝑏 are
𝜙𝑎 = 𝛾 − +𝜋
2 , 𝜙𝑏 = 𝛽 − 𝛾 +
𝜋
2. (8)
From the torque 𝜏𝑎 applied to the actuation link, 𝑓𝑎 can be
obtained as follows:
𝑓𝑎 = 𝜏𝑎√(𝑝 𝑥 − − 𝑙𝑑)2 + 𝑝 𝑦
2 . (9)
As the moment equilibrium around 𝑝𝑏2 is 𝑓𝑡𝑑𝑡 = 𝑓𝑎𝑙𝑑 sin𝜙𝑎,
the fingertip force 𝑓𝑡 is
𝑓𝑡 = (𝑙𝑑 sin𝜙𝑎
𝑑𝑡) 𝑓𝑎. (10)
It can be rewritten by a vector as follows:
𝑓𝑡⃑⃑⃑ = [−𝑓𝑡 cos 𝛾 , −𝑓𝑡 sin 𝛾]𝑇 . (11)
From the force equilibrium 𝑓𝑡 = 𝑓𝑎 𝑐𝑜𝑠 𝜙𝑎 + 𝑇𝑏 𝑐𝑜𝑠 𝜙𝑏 , the
tension of the belt can be calculated as follows
𝑇𝑏 =𝑓𝑡−𝑓𝑎 𝑐𝑜 𝜙𝑎
𝑐𝑜 𝜙𝑏. (12)
As the belt tension and the pressure has relationship 𝑇𝑏 = 𝑟𝑃𝑏,
the pressure 𝑃𝑏 is
𝑃𝑏 = 𝑇𝑏/𝑟. (13)
The total force cause by the pressure of the belt can be easily
calculated by integrating the pressure 𝑃𝑏 from 𝑝𝑐1 to 𝑝𝑐2. The
resultant force vector is
𝑓𝑏⃑⃑ ⃑ = [−𝑇𝑏(sin 𝛽 + sin 𝛼) , 𝑇𝑏(cos 𝛽 − cos𝛼)]𝑇, (14)
and the magnitude is
𝑓𝑏 = 𝑇𝑏√(sin 𝛽 + sin 𝛼)2 + (cos 𝛽 − cos 𝛼 )2. (15)
Finally, from (11) and (14), the contact force on the palm 𝑓𝑜
can be calculated. Because the sum of all the y-direction
elements of the forces 𝑓𝑡, 𝑓𝑏, and 𝑓𝑜 should be zero and 𝑓𝑜 has
only y-direction force
𝑓𝑜 = 𝑓𝑡 sin 𝛾 + 𝑇𝑏(cos 𝛼 − cos 𝛽). (16)
In conclusion, (11), (14), and (16) show the overall force
and pressure distribution. Figs. 6(a)-(f) show the simulated
force and pressure distribution for the several circular objects
with the diameters 40 – 140 mm, where , 𝑑𝑏 , 𝑙𝑑, 𝑙𝑎, and 𝑙𝑡 were set to 21.5, 80, 23, 80, and 40 mm considering the actual
design, respectively. For small objects, as shown in Fig. 6(a),
the belt pressure is the dominant grasping force which is
desirable to deal with the object. In most cases, the force and
pressure are well distributed and the gripper provides stable
form closure compared to other cases briefly illustrated in Fig.
(2). But, when grasping the 140mm diameter object, the palm
(a) (b)
(c)
Fig. 5. (a) Pose of the finger grasping a circular object. (b) Equilibrium pose
of the fingertip and the trajectory of 𝑝 . (c) Free body diagram of the
fingertip.
Trajectory of 𝑝 𝑑𝑡
𝑝
𝑑𝑡
𝜙1
𝜙1
𝜙2
𝜙2
𝛼
𝛽𝛾
𝑝
𝑝𝑐1
𝑝𝑐2
𝑝𝑡
𝑑𝑡
𝑟𝑝𝑐𝑥
𝑝𝑏1
𝑝𝑏2
𝑝𝑜
𝑃𝑏
𝑑𝑡
𝑓𝑡
𝑓𝑜
𝑝𝑎1
𝑝𝑎2
𝑝
𝑑𝑡
𝑓𝑡
𝑓𝑎
𝑇𝑏
𝜙𝑏
𝜙𝑎
𝑝𝑏2
𝑙𝑑
contact force 𝑓𝑜 becomes to negative value. Fig. 6(e) shows
the changes of the forces according to the object diameter and
it also shows that the palm loses contact force from a 124.1
diameter objects. This may not mean that it loses grasping
stability. It can move to the different grasping mode with 2
contact areas, which will be analyzed in Subsection III.C.
Another failure can occur when the contact point of the
fingertip leaves the fingertip surface. but, in Fig. 6, the contact
point resides almost at the center of the fingertip surface for all
object sizes. This property is due to the shape of the slot. For
instance, if the direction of the slot is inclined to one side, the
contact point will move to the distal or proximal ends.
C. Two-Area Contact Case
When palm loses contact, only the fingertip force and the
belt pressure grasp the object. Therefore, these two forces
should be balanced in the y-direction. Considering the distance
between the palm surface and the object, named 𝑑𝑜 as shown
in Fig. 7(a), , 𝑓𝑡⃑⃑⃑ , 𝑓𝑏⃑⃑ ⃑ and 𝑓𝑜 of (6), (11), (14) and (16) can be
re-calculated. But these equations are functions of 𝑑𝑡 as well
as 𝑑𝑜. By solving the following constrained optimization, the
feasible force and contact location can be obtained.
max 𝑑𝑡, 𝑑𝑜
(𝑑𝑡 , 𝑑𝑜) (17)
subject to 𝑓𝑜 = 0, 0 < 𝑑𝑡 < 𝑙𝑡 , 0 < 𝑑𝑜.
Fig. 7(b) shows a simulation result about grasping a 126.4 mm
diameter object. The contact area and the force from the belt
are small, and it is in the vicinity of the transition of grasping
mode. The object bigger than about 127 mm diameter will be
grasped by parallel pinching pose. Fig. 7(c) shows the close-up
view of the force graph of Fig. 6.(e) considering 2 region
contact. For the object with a diameter greater than 124.1 mm,
palm contact force remains zero, and the belt force also decays
to zero until grasping mode transit to the parallel pinching.
IV. MECHANICAL DESIGN AND IMPLEMENTATION
Fig. 8(a) shows the size and shape of the developed BLT
gripper. It has 3-fingers and 5 motors. Each finger has one
motor for flexion. To control the fingertip angle, a single
motor placed at the palm frame adjusts the length of all the 3
belts. The last motor changes the orientation of the two side
fingers simultaneously. Figs. 8(b)-(e) show various poses
taken by actuating this orientation motor. If it rotates the
fingers 180 deg as shown in Fig. 8(c), it can perform a parallel
pinch or compliant grasp of cylindrical objects and also can
perform a clasped pose. By completely folding the center
finger and rotating the side fingers to 90 deg as shown in Fig.
8(e), 2-finger pinching is also available.
Fig. 9 illustrates the detail design of the finger and palm
mechanisms. To increase holding force and save power,
non-backdrivable worm gears were used for finger flexion as
shown in Fig. 9(a). To achieve additional gear ratio, a spur
gear set is used along with the worm gear. The motor for the
belt actuation is placed at the palm frame as shown in Fig. 9(b).
It actuates the 3 lead screws via a gear train and the sliders are
connected to the lead screw nuts. For the finger rotation, the
rotation motor placed at the palm frame delivers the motion
through another gear train as shown in Fig. 9(c), which is a
commonly used configuration for 3-finger grippers including
BarrettHandTM [24][6][9]. Five identical 40 Watt ECX16
motors of MAXON Co. are used for actuation. The total
(a) (b)
(c)
Fig. 7. (a) Pose of the finger in the case of two region contact (𝑓𝑜 = 0).
(b) Simulation result for Dia. 126.4 mm object. (c) Closed up view of the
force graph.
𝜙11
𝜙12
𝜙2
𝜙2
𝛼𝛽
𝛾
𝑝
𝑝𝑐1
𝑝𝑐2
𝑝𝑡
𝑑𝑡
𝑟𝑝𝑐𝑥
𝑝𝑏1
𝑝𝑏2
𝑝𝑜
𝑃𝑏
𝑑𝑡
𝑓𝑡
𝑑𝑜
𝑓𝑡 = .0
𝑓𝑏 = .
𝑓𝑜 = 0𝑑𝑜 = . 0
Dia. : 126.4 mm
Fo
rce
[N]
Object Dia. [mm]
3 region contact
2 region contact
Fig. 6. (a)-(f) Force and pressure distribution for circular objects with the
diameters 40, 60, 80, 100, 120 and 140 mm. (g) Force changes according to
the object diameter.
(a) (b)
𝑓𝑡 = .22𝑓𝑜 = .
𝑓𝑏 = . 𝑓𝑡 = .
𝑓𝑜 = 2 . 2
𝑓𝑏 = 22. 0
Dia. : 40mm Dia. : 60mm
(d)(c)
𝑓𝑡 = .20
𝑓𝑏 = . 0
𝑓𝑜 = .
Dia. : 100mm
𝑓𝑡 = .
𝑓𝑏 = .
𝑓𝑜 = −2.
Dia. : 140mm
124.1
Fo
rce
[N]
Object Dia. [mm](e)
weight is 1.2kg. A summary of specifications is presented in
Table I.
V. EXPERIMENTAL VALIDATION AND DISCUSSION
To verify the performance of the proposed gripper, grasping
tests using various types of objects were conducted. Fig. 10
shows snapshots of the active and continuous transition from
the precise parallel pinching mode to the compliant grasping
mode, and again to the parallel pinching mode. The gripper
picks up an 8 mm diameter pen by parallel pinching with the 3
fingertips as shown in Figs. 10(a) and (b). By actuating the belt
motor with proper coordinated motions of the 3 flexion motors,
the gripper moves the object to a proper position to be grasped
(Fig. 10 (c)). Note that the contact point is distal to the critical
line which means that the object is still in the parallel pinching
region. This mode is desirable when adjusting the object
position without being affected by compliant motions. By
actuating the flexion motors, the object moves in the compliant
grasping region and can be firmly grasped as shown in Figs.
10(d) and (e). By maintaining fixed fingertip angles, the
opening of the fingers makes the object return to the parallel
pinching region (Fig. 10(f)). Finally, by coordinate motions of
the flexion motors and belt motors, the object returns to the
parallel pinching pose as shown in Fig. 10 (g).
In order to prevent over-compression of the object when
changing the fingertip angle (between Figs. 10(b) and (c)), the
coordinated motions of the belt and the link are required. As
illustrated in Fig. 11(a), the width of the object 𝑜 from the
centerline can be obtained as follows:
𝑜 = + 𝑙𝑎 cos 1 (18)
where 1 is the flexion angle in the parallel pinching mode. By
rotating the fingertip around a desired point 𝑝𝑟 which is placed
at the distal position from the contact points of the object
(red-colored points in Figs. 11(a) and (b)), the fingertip angle
can be safely changed without excessively squeezing the
object. If the desired fingertip angle is 𝜙𝑡, as can be seen in Fig.
11(b), the relationship between 𝑜 and the required flexion
angle 2 is as follows:
𝑜 = + 𝑙𝑑 + 𝑙𝑎 cos 2 − 𝑙𝑑 cos 𝜙𝑡 − 𝑑𝑡 sin𝜙𝑡
where 𝑑𝑡 denotes the distance between 𝑝𝑟 and 𝑝𝑏2. Thus,
2 = acos( 𝑜 − − 𝑙𝑑 + 𝑙𝑑 cos𝜙𝑡 + 𝑑𝑡 sin 𝜙𝑡)/𝑙𝑎. (19)
From Fig. 11(b), the position of 𝑝𝑏2 can be calculated as:
𝑝𝑏2 = [ 𝑏2 𝑏2
] = [ + 𝑙𝑑 + 𝑙𝑎 cos 2 − 𝑙𝑑 cos 𝜙𝑡
𝑙𝑎 sin 2 − 𝑙𝑑 sin 𝜙𝑡
]
Therefore, the required belt length 𝑑𝑏2 to rotate the fingertip to
𝜙𝑡 is calculated as follows:
𝑑𝑏2 = 𝑝𝑏1𝑝𝑏2̅̅ ̅̅ ̅̅ ̅̅ ̅ (20)
= √(𝑙𝑑 + 𝑙𝑎 cos 2 − 𝑙𝑑 cos𝜙𝑡)2 + ( 𝑙𝑎 sin 2 − 𝑙𝑑 sin𝜙𝑡)
2
Consequently, by continuously actuating the flexion and belt
actuators in accordance with (19) and (20), the fingertip angle
can be safely changed without squeezing or damaging objects.
On the other hand, as shown in Fig. 12, the BLT gripper also
can directly perform the compliant grasping without going
through the parallel pinching mode. By actuating flexion
actuators with the fingertips inclined, the gripper can scoop the
object to become the compliant grasping mode.
Fig. 13 shows several examples of grasping experiments of
various types of objects. Figs. 13(a) and (b) show the grasping
with the 3 fingers oriented symmetrically around the objects.
The successful change in grasping mode is shown in these
Fig. 8. (a) Size and shape of BLT gripper. (b) Clenched pose. (c) 3-finger
2-side parallel pinching. (d) 3-finger point symmetrical parallel pinching
pose. (e) 2-finger parallel pinching.
91
80
40
20
8
81
23
(a)
(b) (c)
(d) (e)
(a) (b)
(c)
Fig. 9. (a) Finger flexion and slider mechanism. (b) Exploded view of the
palm frame and the finger part. (c) Finer rotation mechanism.
Grasping Motor
Belt motor
Worm and worm wheel
Spur gears
Lead screw
Gear train
Slider
Belt motor
Palm frame
Lead screw nuts
Rotation motor
Gear train
Palm frames
Table I. Specification of BLT gripper
Size and weight 81 208 96 mm (WHT), 1.2 kg
No. of actuators Total 5 (Flexion 3, belt 1, rotation 1)
Range of motion
Flexion -30 ~ 90 deg
Rotation 0 ~ 180 deg
Fingertip1) 0~ 60 deg
Fingertip force
Peak force 60 N
Holding force 60 N
Cont. force 10 N
Gear ratio
Flexion 125 : 1
Rotation 257.6 : 1
Fingertip 172.7 : 1
1) Varies according to the finger pose
figures. In Fig. 13(c), the active transition of the grasping
mode was unavailable because the fingertips are in contact
with the middle of the flat surface of the object, and thus the
fingertips cannot move under the object. It is a disadvantage
compared to High-DOF dexterous hands which can perform
in-hand manipulation using finger gaits. Figs. 13 (f)-(n) show
snapshots from grasping experiments performed with various
irregularly shaped objects. As can be found in Figs. 13(f) and
(g), the motion allowed by the straight slot design enables the
belts and fingertips to conform to the shape of the objects.
To observe the force and pressure distribution, pressure
measurement was performed on several cylindrical objects.
Fig. 14 (a) shows an experimental setup using Pliance Pressure
Mat from Novel Co. Fig. 14(b) shows the pressure distribution
on the center finger, where each cell is 2.52.5mm and the
number in each cell represents the pressure in unit kPa. The
results from grasping the 40 and 60 mm diameter objects show
that the belt provides wide and even pressure distribution for
the small objects as derived in Subsection III.B. When
grasping the 80 mm diameter object, fingertip and palm forces
take substantial contact force. In the case of the 100 mm
diameter object, the fingertip force becomes dominant, which
implies that it is near the verge of the parallel pinching mode.
Due to the limited accuracy of the pressure mat, exact
comparisons with the theoretical values were not performed.
For the verification of precision, a repeatability test of the
fingertip motion was performed. As shown in Fig. 14(c), 30
iterations of fingertip distance were measured and the
maximum deviation from the average distance was 0.0321 mm.
This precision is accomplished by the sufficiently large
parallel 4-bar structures and flexible belt materials with
high-stiffness.
The BLT gripper aims to achieve grasping mode adjustment,
a goal that has also been studied with the iHY hand the Velo
gripper. The iHY hand has 3 fingers and 5 actuators like the
BLT gripper [9]. Each finger has one flexor tendon to achieve
under-actuation. The fourth actuator controls the coupled
rotational motion of two fingers, similar to the BLT gripper.
To increase the manipulation capability, an additional
extension tendon actuated by the fifth actuator was added to
one finger of the iHY Hand. In contrast, the BLT gripper
utilizes one actuator to adjust all three fingertip angles
simultaneously to achieve active grasping mode change while
maintaining the parallel 4-bar structure for precision. It is
notable that iHY hand has flexure joints instead of revolute
joints. This enables remarkable robustness against collisions at
the expense of a certain level of precision, which is a clear
difference from the design philosophy of the BLT gripper. The
Velo gripper also has both capabilities of the parallel pinching
and compliant grasping [17]. To maintain the parallel finger
angles, coupling tendons instead of the parallel 4-bars were
used. It can be advantageous for a wide range of motion but the
precision can be lower than the 4-bar mechanism. The
grasping mode of the Velo gripper is also determined by the
contact point with objects, while the BLT gripper can actively
change the mode by using one more actuator. Another
difference between the BLT gripper and these other grippers is
that the contact occurs on a surface of the belt in tension with
uniform pressure not concentrated on a point or line. This is a
desirable property for compliant grasping.
VI. CONCLUSION AND FUTURE WORK
This paper presented a 3-finger 5-DOF adaptive gripper that
has active transition capability between the precise parallel
pinch and compliant grasp. The simple and unique finger
mechanism provides an appropriate grasping pose with stable
form closures and applies well-distributed grasping force and
pressure to the objects with the arbitrary size. The gripper
Fig. 11. Coordinated flexion and slider motions for the
fingertip rotation to prevent squeezing the object with
excessive force.
1𝑝𝑏1
𝑙𝑎
𝑙𝑑
𝑑𝑡
𝑝𝑏2
𝑝𝑟
𝑜
𝑑𝑏1 = 𝑙𝑎 𝑑𝑏2
2𝑝𝑏1
𝑙𝑑
𝑑𝑡
𝑝𝑟
𝑜
𝑙𝑎
𝑝𝑏2𝑙𝑑
𝜙𝑡
(b)(a)
Fig. 12. Snapshots of direct compliant grasping of the 8 mm
diameter pen without mode transition.
(a) (b) (d)(c)
Open pose with inclined fingertips
Direct compliant graspingusing flexion actuators
Fig. 10. Snapshots of active transition between precise parallel pinching and compliant grasping of a 8 mm diameter pen. After
grasping, the gripper returns to the parallel pinching pose.
Parallel pinching Inclining fingertip
Parallel pinching region
Compliant Grasping using the belt
Compliant grasping region
Transition to parallel pinching pose
Compliant grasping region
(a) (b) (d)(c) (e) (f) (g)
mechanism was carefully designed with the consideration of
practical applications and future commercialization; using the
minimum number of the actuators maintaining enough
capability of various grasping tasks, achieving high payload
and durability, and active transition of the grasping mode
without complicated control algorithm. The force and pressure
analysis and the experimental results verified the grasping
performance and active transition capability.
For the future work, robust and reliable joint torque sensors
and fingertip force sensors will be developed. Improvement
including additional weight and size minimization will be
carried out with the aim of being widely used as a versatile tool
to accelerate the progress in field of robot manipulation.
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[24] https://advanced.barrett.com/barretthand
Fig. 13. (a) and (b) Active transition with cylindrical and spherical objects. (c) Pinching and grasping of a cylindrical object where the grasping mode was
manually changed. (d) and (e) 2-finger parallel pinching. (f)-(m) grasping irregular shaped and various sided objects. (n) grasping and operating a hand drill.
(a) (b) (c) (d)
(e) (f)
Dia.60mm
Dia. 120 mm
(g) (h)
(i) (j) (k) (l) (m) (n)
(a) (b)
(c)
Fig. 14. Measurement of the pressure for cylindrical object grasping. (a)
Experimental setup (Pliance Pressure Mat, Novel Co.). (b) Measured data
for diameters 40, 60, 80, and 100 mm (Cell size: 2.5 mm2 , unit: KPa). (c)
Repeatability test using a dial gauge (Mitutoyo Co. resolution 0.001 mm).
Fin
ger
tip
Bel
tP
alm
Pressure mat
Finger under measurement Diameter: 40 mm 60 mm 80 mm 100 mm
Average
(mm)4.153
Std.
deviation
(mm)
0.0116
Max.
deviation
(mm)
0.0321
Dis
tanc
e (m
m)
Iteration
4.1
4.12
4.14
4.16
4.18
4.2
0 5 10 15 20 25 30